Locals indicate mess made after last boat entered river (6)
I believe the answer is:
thames
'river' is the definition.
(English river)
'locals indicate mess made after last boat entered' is the wordplay.
I cannot really understand how this works, but
'last' could be 't' (I've seen this in other clues) and 't' is found in the answer.
'boat' could be 'e' and 'e' is found in the answer.
The remaining letters 'hams' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for thames that I've seen before include "Shipping forecast area" , "It flows through London" , "River of southern England" , "Capital's river" , "'Old Father ___'" .)